Plant is a natural source providing mankind various secondary metabolites with antiviral activity, which provides a new way for control of plant virus diseases. Using local lesion and leaf discs assay, extracts from 164 medicinal plant species belonging to 78 families in China were screened for antiviral activity against Tobacco mosaic virus (TMV). Of the plants tested, two plant species, i.e. Ailanthus altissima and Brucea javanica, could significantly inhibit virus infectivity and replication. Antiviral bioactivity, mechanism and active compounds of these two medicinal plants were studied. Extract condition, stability, formulation and the control effect on vector insect of antiviral substances extracted from two medicinal plants were also researched in this paper. The main results were as follows:1. Antiviral activity of the 164 medicinal plant species against TMV was screened by means of local lesion and leaf discs assay. Most of medicinal plants exhibited certain antiviral activity of inhibiting virus infection. 21 plant species including Ailanthus altissima and Brucea javanica could effectively inhibit virus infection. The results also indicated that 11 plant species significantly inhibited the replication of virus, and among them Ailanthus altissima and Brucea javanica showed higher inhibition.2. Ailanthus altissima and Brucea javanica were evaluated for not only the antiviral effect on virus infection and replication, but also the therapeutic effect on tobacco mosaic disease caused by TMV, respectively. The results demonstrated that both Ailanthus altissima and Brucea javanica could significantly inactivate TMV, CMV and PVY in vitro assay. When tested at a concentration of 1mg/mL, they produced more than 90% inhibition of virus. Determination of the inhibition of TMV replication with leaf-discs method showed that Ailanthus altissima and Brucea javanica had a high inhibitory activity against TMV replication in leaf discs of Nicotiana tabacum CV, K_(326). Within 12-36 hours after inoculation, extracts from Ailanthus altissima and Brucea javanica could effectively suppressed virus replication. Six-eight hours after inoculation, the inhibitory rate of Ailanthus altissima and Brucea javanica (lmg/mL) was 89.4% and 90.9%, respectively. The experimental results in greenhouse and in the field indicated thatthe extracts from Ailanthus altissima and Brucea javanica had some certain therapeutic effect on tobacco mosaic disease caused by TMV in tobacco plant.3. The extracts from Ailanthus altissima and Brucea javanica didn't have direct viricidal effect on TMV particles, while they showed strong activity reducing TMV-RNA infectivity and could affect the polymerization and antigenic determinant of TMV coat protein in vitro. By determining the activity of the defendant enzymes including POD, PPO, PAL and SOD involved in the interaction between virus and host, the content of chlorophyll and MDA, and by observing the isoezyme spectra changes of POD and PPO, it was found that extracts from Ailanthus altissima and Brucea javanica could induce resistance to TMV in the host to some extent, increase inhibitory activity of host against virus infection and reduce the damage caused by virus infection to host cells.4. By bioassay-guided fractionation, active constituents from Ailanthus altissima and Brucea javanica were isolated and purified through solvent extraction, systemic solvent separation, silica gel column chromatography and recrystalization. The structure and the bioactivity of purified compounds were also studied. Four active constituents (Cl, C2, C3, C4)showing certain antiviral activity were isolated from Ailanthus altissima. Two active constituents (Yl and Y2)with strongly inhibitory activity against TMV infection and replication were isolated from Brucea javanica. The results of bioassay indicated that active constituent. i.e.Yl, showed highest antiviral effect on TMV among six active constituents. Chemical structure of Yl was elucidated to be Brucein D by means of chemical reaction. UV, MS, IR,'H-NMR. 13C-NMR, and comparison with the spectral data of the known compounds. The structures of other active constituents were also analyzed by GC-MS, HPLC-MS and IR techniques.5. To effectively extract antiviral substances from Ailanthus altissima and Brucea javanica. the extraction process was evaluated by orthogonal design(Lq (34)). The way that 95% ethanol was used to extract, adding 7 times ethanol with 2h of extracting time was the optimum extraction process for Ailanlhus allissima. The optimum extraction condition for Brucea javanica was composed of 5 times 95 % ethanol with lh of extracting time. The stability such as the resistance to sunlight, PH and temperature were investigated. The results showed that antiviral